Does Mac allow summer school ? I already have my 6 Uni credits, I just need to upgrade a mark :/

If you're referring to high school, marks from this summer are not taken into account for admissions, because the university doesn't receive them in time. You can take a course if you need a credit as a course prereq or something, but if your average is too low and you want to upgrade, then the summer course won't count until next year.

Are you first year student or a prospective student? If you are going into second year, i think the deadline to sign up for summer school has passed already.
http://registrar.mcmaster.ca /regis...sessional.html

If you're a high school student, call the registrars and see if it's possible to upgrade your high school marks for admission purposes.
http://registrar.mcmaster.ca /contacts.html